1. Field of the Invention
To provide easy steering, sure handling, smooth operation and good tire wear, wheel alignment factors, such as caster, camber and toe angles for the front and rear wheels, must be considered. Conventionally, such wheel alignment factors are measured for all four wheels of the vehicle, while the wheels are stopped, in a wheel alignment testing device. The wheel alignment is then desirably adjusted. Such a wheel alignment measuring method and apparatus, however, typically can not provide an automotive vehicle with steering characteristics which are adequately corrected, so that the vehicle travels straight, by adjusting only toe angles of the front and rear wheels, since measurements of the wheel alignment vary, depending upon measuring points of the wheels.
2. Description of Related Art
In recent years, it has become more popular to measure the wheel alignment of automotive vehicles while all the front and rear wheels continuously rotate. The front and rear wheels are supported, in a horizontal plane, by front and rear roller or rotary drum units, driven, respectively, by electric motors. Each roller unit has at least a pair of rollers having horizontal axes of rotation parallel to each other and drives the wheels so as to measure wheel alignment factors, such as camber and toe angles, in a condition which is substantially the same as an actual traveling condition. This allows the wheel alignment factors to be adjusted based on measurements obtained under traveling conditions which approximate actual conditions. Such an apparatus is known from, for instance, Japanese Unexamined Patent Publication No. 57 - 53613. The measurement and adjustment of wheel alignment may alternatively be simultaneously performed while the wheels continuously rotate. Such an apparatus is known from, for instance, Japanese Patent Application No. 2 - 91370, filed by the same applicant as that of this application.
As is well known, roads are designed to have a crown providing an appropriate cant so that water will drain off and be removed from the road surface. Due to such a cant of the road, the automotive vehicle has a lateral weight component which is approximately 2 to 3 per cent of its total weight, which causes an inclination of the automotive vehicle towards that side of the road which is downwardly inclined. For this reason, tires have recently been designed to generate a lateral stress (which is, generally, referred to as a lateral residual stress), approximately equivalent to one fourth (if they are for four wheel vehicles) of the lateral component of weight, at a shoulder portion thereof. Such tires help to keep the automotive vehicle travelling straight forward so that it is almost free from effects provided by the cant of the road.
However, in a conventional wheel alignment apparatus, in which the wheels of an automotive vehicle are disposed in a horizontal plane even though the wheels of the vehicle continuously rotate while measuring the wheel alignment of the automotive vehicle, only the lateral residual stress is reflected in measuring the wheel alignment, and the lateral component of weight of the automotive vehicle is ignored. Accordingly, the toe angles of the wheels tend to be over-corrected if the wheels are adjusted until the tendency of movement of the automotive vehicle towards the left, if the vehicle is a left-hand drive car, or towards the right, if the vehicle is a right-hand drive car, disappears. An automotive vehicle with the toe angles of the wheels thus over-corrected is apt to slide towards the downwardly inclined side of the road due to the lateral component of weight thereof, even though the tires have a proper lateral residual stress, when the vehicle travels on an actual road with a certain cant. The automotive vehicle, therefore, will not properly travel straight.
In order to assure that the automotive vehicle will be stable and travel straight forward, the wheel alignment can be corrected by taking lateral residual stresses of the tires into consideration. However, since tires have lateral residual stresses which are largely different from one another, due to typical manufacturing errors, it is hard to take the lateral residual stresses of the tires into consideration in correcting wheel alignment.